The present invention relates generally to methods and knives for shearing a metal bar, and more particularly to methods and knives for shearing a tapered end on a metal bar as it undergoes hot rolling.
In the hot rolling of a metal bar such as steel, the bar, in a semi-finished shape such as a billet, is heated in a furnace and then typically moved continuously through a succession of spaced-apart rolling mill stands in each of which the bar undergoes a reduction in cross-sectional area and an accompanying increase in length. During the hot rolling operation, certain problems may occur. For example, in a typical rolling mill stand, there are a pair of vertically spaced rolls, separated by a gap, and a device is provided for guiding the front or downstream end of the bar into the gap between the two rolls. Occasionally, the downstream end of the bar may jam in the guide device, or it may strike a part of a roll called the "roll collar." In either case, those portions of the bar upstream of the downstream end back up and become entangled resulting in what is known as a "cobble." When this occurs, the entire mill must be shut down until the cobble can be removed, and the cobbled metal bar must be scrapped.
Another problem which may occur is the tendency of the downstream end of the bar to open up when leaving a rolling stand, resulting in what is known as a "split end". The split end may be too wide to fit through the gap between the rolls at the next mill stand, and this causes cobbling to occur when the split end encounters those rolls.
After a bar has undergone part of a continuous hot rolling operation, the front two to four feet of the bar are usually sheared off utilizing a conventional shearing device which typically cuts through the bar along a plane. The front or downstream end portion of the bar is cut off because it cools faster than the upstream parts of the bar, and this cold end portion can cause problems if not removed. For example, because the downstream end portion of the bar cools faster than trailing portions of the bar, the downstream end portion becomes harder than the upstream portions of the bar, and, when the hard downstream portion engages a roll, it may cause marking to occur on the roll. This, in turn, causes the marked roll to roll defects into the trailing part of the bar and on subsequently rolled bars.
Shearing off the downstream end portion of the bar also tended to reduce split ends, but a substantial amount of split end formation still occurred when the downstream end portion of the bar was sheared in accordance with conventional practices.